This invention relates to a hydraulic system for supplying hydraulic fluid to a plurality of hydraulic drive motors found in heavy equipment, such as excavators, backhoes and the like.
In recent years, the use of hydraulic drive systems for controlling the various functions in heavy equipment, for example, vehicles having earthworking implements, such as excavators, has become more common. For example, quite recently, completely hydraulic systems have been developed for use with heavy duty vehicles such as excavators.
In such systems, the actuation of control valves that control the flow of fluid to the various drive motors is accomplished by hydraulic fluid. One example of such a system incorporates a main hydraulic circuit that includes one or more main pumps and a plurality of hydraulically actuated valves that control the flow of fluid between the pumps, a reservoir and hydraulic motors associated therewith. Typically, the valves are self-centering and are opened by a control circuit that is capable of supplying small amounts of fluid under pressure from a source to opposite ends of the valves for actuating the valves. The fluid flow in the control circuit is controlled by manually and electrically operated valves and acts as a "pilot system" for actuating the main control valves.
Such a system has a number of advantages, the primary one being that the function that is being performed can be accurately controlled. For example, utilizing the "pilot system" for actuating the main valves gives the operator the ability to introduce very small amounts of fluid to the hydraulic motors.
While such systems have found a remarkable degree of success, they are susceptible of improvements to provide additional capabilities, thereby enhancing their usefulness, desirability and potential for commercial acceptance and success.
For example, because of the power and torque often required to move such vehicles along the ground, they tend to move relatively slowly. This slow movement is also advantageous in facilitating maneuverability.
There are occasions when it would be desirable to increase the maximum speed of travel, e.g., when it is desired to travel over extended distances. Under these circumstances, maximum torque or power is not usually required. In addition, when traveling long distances, it would also be desirable to relieve the operator from maintaining pressure on a manual control lever in order to sustain movement, which can result in operator fatigue and may also limit the operator's ability to perform other functions simultaneously.